1. Field of the Invention
The present invention relates to a recording and reproducing apparatus having a fixed distortion correcting function and more particularly, to an apparatus having a function to correct distortion specific to the recording and reproducing apparatus to record information by irradiating a holographic recording medium with an information light beam and a reference light beam at the same time.
2. Description of the Related Art
According to the holographic recording and reproducing apparatus, a predetermined position of a holographic material layer of a holographic recording medium is irradiated with an information light beam corresponding to two-dimensional page data and a reference light beam at the same time to record the two-dimensional page data.
The two-dimensional page data is recorded as an interference pattern of the two light beams (information light beam and reference light beam).
Meanwhile, the medium is irradiated with only the reference light beam and its reflected light beam (referred to as reproduction light beam also) is detected by a two-dimensional image sensor (CCD) and the page data recorded on the medium is reproduced.
As one of documents disclosing and reproducing apparatus, there are known Japanese Unexamined Patent Publication No. 2002-216359 and Japanese Unexamined Patent Publication No. 2004-158114.
Although the holographic recording and reproducing apparatus includes many optical components such as a light source to emit a laser beam, a lens group, a spatial light modulator (SLM), a mirror group, a two-dimensional image sensor (CCD) and the like, there is a manufacture error in each component in the apparatus.
Because of this error, fixed distortion specific to the apparatus is generated due to a difference in optical characteristics or a noise at the time of recording or reproducing.
Representative fixed distortion includes distortion aberration and a low-frequency noise.
For example, according to an objective lens provided in the vicinity of the medium, it is impossible to manufacture the same lens having ideal optical characteristics all the time, so that lens aberration exists in a product.
The lens aberration causes distortion aberration shown in FIG. 19 at the time of reproducing. This distortion aberration varies a position of a detection pixel of a reproduction signal.
FIG. 19(a) shows an example of a reel-shaped distortion and FIG. 19(b) shows an example of a barrel-shaped distortion.
Here, a rectangle shown by dotted line in FIG. 19 is an ideal two-dimensional image configuration detected by the two-dimensional image sensor (CCD) normally.
FIG. 19(a) shows a case in which a configuration of a detected image which should be a rectangle originally is drawn out and distorted due to aberration of a lens group including an objective lens.
FIG. 19(b) shows a case in which a configuration of a detected image which should be a rectangle originally is drawn in and distorted due to aberration of a lens group including an objective lens.
Although the central part of the CCD is hardly distorted and data is correctly reproduced, the data cannot be correctly reproduced toward the periphery (four corners especially) of the CCD in either case.
In addition, there is a case where a low-frequency noise is generated as shown in FIG. 20 due to sensitivity offset of the CCD or lens aberration.
The low-frequency noise means variation in luminance of the reproduction light beam detected on the CCD.
That is, a different luminance value is generated from each detection pixel at the time of reproducing.
For example, when it is assumed that all data which should be detected are the same data (1), although a reproduction light beam having the same luminance corresponding to the data (1) should be detected all pixels by the CCD, when there is lens aberration and the like, the light whose luminance should be determined as “1” could be detected as the light whose luminance is shifted to “0”.
That is, even when it is determined that all data is the same correctly, the luminance detected in each pixel is varied actually in some cases. In addition, when the variation in luminance is large, the luminance which should be detected as “1” is detected as “0”, which cause a reproduction error.
A correction method called “adaptive equalization” is proposed to correct the low-frequency noise shown in FIG. 20 so that data in which luminance is not varied can be reproduced.
The adaptive equalization is a method of removing the low-frequency noise by a kind of high-pass filter such as a FIR filter used in an optical disk and the like, in which a filtering coefficient is not fixed and needs to be updated as needed.
In addition, the adaptive equalization is performed when the user data is reproduced and its calculation amount is excessive, so that a circuit size to implement the adaptive equalization is large and it takes time for a correcting process.
According to a reproducing process of the holographic recording and reproducing apparatus, since the reproducing process is performed using the two-dimensional page data having great amount of data as a unit, when the adaptive equalization is used, it takes time for the correcting process, so that the reproducing process could not be performed in a practical time.
In addition, when such correcting process is implemented by hardware, a circuit size becomes large and a cost becomes high.